Japanese Unexamined Patent Application Publication No. 2009-078735 seeks to provide a driving assistance device capable of controlling lane changing safely, even when there is no space into which lane changing can be performed ([0005], Abstract).
In order to achieve this, the driving assistance device of Japanese Unexamined Patent Application Publication No. 2009-078735 (Abstract) includes an assistance start section that starts lane change assistance, a detection section that detects a relative distance and a relative speed between an own-vehicle and another vehicle, and a computation section that, based on the relative distance and relative speed, computes a collision risk degree with the other vehicle when the own-vehicle to change lanes. The driving assistance device further includes a first determination section that determines the permissibility of lane changing based on the relative distance, the relative speed, and the collision risk degree; a decision section that decides on a target space for lane changing based on the relative distance and relative speed in cases in which lane change cannot be performed; and a second determination section that determines whether or not a space that allows lane change exists in the target space. The driving assistance device further includes a setting section that sets a target speed toward a standby position for lane change when such a space does not exist, and that sets a target speed toward a position where lane changing is possible when such a space does exist; and a controller that controls the speed of the own-vehicle so as to become the target speed.
In Japanese Unexamined Patent Application Publication No. 2009-078735, specifically, the permissibility of lane change is determined based on an inter-vehicle distance and a time-to-collision (TTC) (FIG. 7, [0033]). In cases in which lane change is not permissible (S250 in FIG. 2: NO), the driver is warned (S300 in FIG. 2, FIG. 9).
In Japanese Unexamined Patent Application Publication No. 2009-078735, in a case in which another vehicle concedes a space for the own-vehicle to change lanes, determination is thus switched to determination that a space allowing safe lane change is present in the target space, and the speed control toward the position where lane changing is possible is started ([0075], FIG. 14(e)).